P0300 on a 2019 Toyota Corolla
P0300 is a generic OBD2 powertrain code for random or multiple cylinder misfire. Unlike P0301 through P0308, it does not point to one specific cylinder, so the cause can be ignition, fuel, air, sensor, or mechanical.
Not your vehicle? See general P0300 info.
Most common causes on the 2019 Toyota Corolla
Ranked from 200 NHTSA owner complaints filed for this vehicle.
- 1
Faulty Coolant Bypass Valve
High severityA defective coolant bypass valve can cause engine overheating, coolant loss, and misfires (P0300) due to internal engine temperature issues or coolant intrusion. Look for 'Engine Maintenance Required' messages, coolant loss without visible leaks, and white exhaust smoke.
Reported in 18 NHTSA complaints - 2
Faulty Fuel Injector(s)
Medium severityA malfunctioning fuel injector can cause a lean or rich condition in a specific cylinder, leading to a random or multiple cylinder misfire. Confirm by checking individual cylinder misfire counters and performing an injector balance test.
Reported in 1 NHTSA complaint - 3
Defective Fuel Pump
Medium severityA failing fuel pump can cause low fuel pressure, leading to fuel starvation and random misfires across multiple cylinders. Confirm by checking fuel pressure and volume against specifications.
Reported in 1 NHTSA complaint - 4
Worn or Faulty Spark Plugs
Low severityWorn out or fouled spark plugs fail to ignite the air/fuel mixture properly, causing misfires. Inspect spark plugs for wear, carbon buildup, or incorrect gap.
Common cause - 5
Faulty Ignition Coils
Medium severityA weak or failing ignition coil will not provide enough voltage to the spark plug, resulting in a misfire. Confirm by swapping the suspected coil to another cylinder and seeing if the misfire follows.
Common cause - 6
Vacuum Leak
Medium severityUnmetered air entering the intake manifold leans out the air/fuel mixture, causing random misfires. Check for cracked hoses, intake manifold gasket leaks, or a faulty PCV valve using a smoke machine.
Common cause
These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0300 on your car still needs shop confirmation before any repair.
Common symptoms
- Flashing or steady check engine light
- Rough idle
- Shaking under acceleration
- Loss of power
- Fuel smell from exhaust
What to check before replacing parts
Scan for cylinder-specific misfire codes
Look for: A companion P0301–P0308 pins the misfire to one cylinder — start there. P0300 with no cylinder code points at something hitting several at once: fuel pressure, a large vacuum leak, or timing.
Inspect the spark plugs and coils
Look for: Pull the plugs: black/sooty = rich, oily = oil intrusion, white/blistered = lean or overheating, cracked porcelain or an out-of-spec gap = replace. Then swap a coil to another cylinder — if a cylinder-specific code follows the coil, the coil is bad.
Check fuel trims for a vacuum leak
Look for: High positive short- and long-term trims (worst at idle, improving as RPM rises) mean unmetered air is getting in. Smoke-test the intake and listen for hissing.
Verify fuel pressure
Look for: Compare running/cranking pressure to the vehicle spec. Low or dropping pressure starves multiple cylinders and reads as a random misfire — suspect the pump, filter, or regulator.
Compression / leak-down test if ignition and fuel check out
Look for: A cylinder 15%+ below the others, or leak-down hissing from the intake (intake valve), exhaust (exhaust valve), or oil filler (rings), means the misfire is mechanical — not a swap-in part.
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